The present invention is generally directed to cellulosic fibers and, more particularly, to carboxylated cellulosic fibers and methods for their formation and use.
The tensile or sheet strength of fibrous products derived from cellulose fibers is due in large part to attractive fiber-to-fiber interactions. These interfiber interactions include hydrogen bonding interactions between fibers having hydrogen bonding sites. For cellulose, hydrogen bonding sites primarily include the hydroxy groups of the individual cellulose chains.
The present invention relates to increasing the strength of cellulosic fiber sheets by incorporating carboxyl groups into cellulosic fibers from which the sheets are made. In accordance with the present invention, carboxyl groups are incorporated into cellulosic fibers through reaction with a carboxylating agent that is a polycarboxylic acid.
Treating cellulosic fibers with polycarboxylic acids is known in the art. For example, polycarboxylic acids have been used as crosslinking agents for cellulose. Cellulose has been modified by reaction with dicarboxylic acids and their derivatives to form simple diester crosslinks. Phthalic, maleic, and succinic anhydrides have been used to form diester crosslinks in cellulose. Cotton has been treated with dicarboxylic acid chlorides having varying chain lengths (e.g., from succinyl to sebacoyl) to provide ester crosslinks. Dicarboxylic acids have also been reacted with cellulose to provide crosslinked cellulose containing diester crosslinks of various lengths (e.g., C3-C22). However, oxalic acid has been shown to be unreactive to cellulose crosslinking, and succinic and glutaric acids have been shown to have only slight reactivity. For a review of ester crosslinked cellulosic fibers, see Tersoro and Willard, CELLULOSE AND CELLULOSE DERIVATIVES, Bikales and Segal, eds., Part V, Wiley-InterScience, New York, 1971, pp. 835-875.
Polycarboxylic acid crosslinked fibers and their preparation and use are also described in U.S. Pat. Nos. 5,137,537; 5,183,707; and 5,190,563, issued to Herron et al. The Herron patents generally describe the preparation and use of individualized, polycarboxylic acid crosslinked cellulosic fibers having advantageous reduced water retention value properties. These fibers have a C2-C9 polycarboxylic acid crosslinking agent reacted with the fibers in the form of an intrafiber crosslink bond. The cellulosic fibers treated with the polycarboxylic acid crosslinking agents are cured at elevated temperature (e.g., about 190xc2x0 C.) to exhaustively couple the polycarboxylic acid to the cellulosic fibers through ester crosslinks. The C2-C9 polycarboxylic acid crosslinking agents include citric acid, 1,2,3-propanetricarboxylic acid, 1,2,3,4-butanetetracarboxylic acid, and oxydisuccinic acid, among others.
Polymeric polycarboxylic acids have also been used to crosslink cellulosic fibers. The use of polyacrylic acid crosslinking agents, including copolymers of acrylic acid and maleic acid, is described in U.S. Pat. No. 5,549,791, issued to Herron et al. These polycarboxylic acid crosslinking agents were found to be particularly suitable for forming ester crosslink bonds with cellulosic fibers. Unlike some conventional crosslinking agents (e.g., C2-C9 polycarboxylic acids such as citric acid) that are temperature sensitive, polyacrylic acid is stable at high temperature and, therefore, can be subjected to elevated cure temperatures to effectively and efficiently provide highly crosslinked fibers. The Herron patent describes curing polyacrylic acid treated cellulosic fibers at about 190xc2x0 C. for about minutes to form interfiber ester crosslinked bonds.
The mechanism of crosslinking paper with polycarboxylic acids has been described. See, Zhou et al., Journal of Applied Polymer Science, Vol. 58, 1523-1534 (1995). Brief thermocuring of paper treated with aqueous solutions of polycarboxylic acids provided paper having excellent wet strength through crosslinking. The effectiveness of a polycarboxylic acid to impart wet strength to paper was found to increase with increasing polycarboxylic acid functionality (i.e., number of carboxyl groups). Butanetetracarboxylic acid (BTCA) was found to be more effective than tricarballylic acid (TCA), which in turn was found to be significantly more effective than succinic acid (a dicarboxylic acid). The excellent wet strengthening properties of polycarboxylic acids such as BTCA and TCA were determined to reflect the acids"" ability to form multiple, reactive anhydrides during the curing reaction either directly, in the form of a dianhydride for BTCA, or in a successive, stepwise mode for BTCA and TCA. For succinic acid, such a consecutive reaction is more difficult and reaction with succinic acid leads to a substituted cellulose having a considerable proportion of single carboxylic acid groups attached to cellulose through an ester link. Because the residual single carboxyl group reacts with cellulosic hydroxyl groups at a slower rate, succinic acid has been shown to be a poor crosslinking and wet strength agent for paper. See Zhou et al.
The mechanism of polycarboxylic acid crosslinking of papers has been shown to occur in four stages: (1) formation of 5- or 6-membered anhydride ring from polycarboxylic acid; (2) reaction of the anhydride with a cellulose hydroxyl group to form an ester and link the polycarbide acid to cellulose; (3) formation of additional 5- or 6-membered ring anhydride from polycarboxylic acids"" pendant carboxyl groups; and (4) reaction of the anhydride with other cellulose hydroxyl groups to form ester crosslinks.
Reaction of paper with succinic acid at 150xc2x0 C. results in the formation of ester bonds or links, the number of which increases with curing time. A small amount of crosslinking is observed, and the amount of crosslinking increases significantly with curing time and higher curing temperatures.
While polycarboxylic acid reaction with cellulose leads to substitution and crosslinking, only interfiber ester covalent bonds can support paper structure when wet. Because the ester links are water stable, the crosslinks prevent swelling of fibers and thus may help hold the paper""s fibers together. Although the introduction of carboxy groups into paper through esterification may affect some aspects of the paper""s characteristics, the paper""s primary wet strength results from the formation of interfiber ester covalent bonds. Both crosslinking and formation of interfiber ester covalent bonds are essentially the same chemical reaction. It can be seen that the critical factors are whether the fibers are in contact with one another during curing and the ability of the polycarboxylic acid to undergo more than one esterification reaction with cellulose hydroxyl groups.
Although the number of carboxyl groups incorporated into a paper treated with succinic acid can be high, the resulting paper has little wet strength. Because these pendant carboxyl groups are largely incapable of further reaction with cellulose""s hydroxyl groups to provide interfiber bonds or crosslinked fibers under normal curing conditions, most of these pendant carboxyl groups remain free. The mere presence of carboxylic acid moieties in a paper""s cellulosic fibers does not impart wet strength to the paper.
However, cellulosic fibers modified to include carboxyl groups have been shown to impart strength to sheets in which the fibers are incorporated. More specifically, fibrous sheets incorporating carboxymethylated cellulose and carboxyethylated cellulose have been found to be relatively easily fibrilated or repulped and formed into sheets having superior strength properties. See U.S. Pat. No. 5,667,637, issued to Jewell et al., and references cited therein.
The wet strength of fibrous sheets made from carboxymethylated and carboxyethylated cellulose can be further increased by blending the carboxylated fibers with a wet strength resin, particularly a cationic additive. See, for example, U.S. Pat. No. 5,667,637, and references cited therein. Generally, the addition of carboxyl groups to cellulose is believed to enhance the efficiency of the wet strength resin by imparting wet strength to fibrous sheets containing such fibers. The combination of carboxyethylated fibers and cationic additive materials has been found to be unexpectedly advantageous with regard to wet strength compared to combinations of carboxymethylated fibers and similar cationic additive materials. See U.S. Pat. No. 5,667,637.
Despite the advances in the use of carboxylated fibers and the formation of fibrous webs incorporating such fibers, there exists a need for carboxylated fibers that do not suffer the drawbacks of carboxymethylated and carboxyethylated cellulosic fibers, which include high cost and lost hemicelluloses. Accordingly, there is a need in the art for modified cellulosic fibers having advantageous absorbent properties and, in addition, having enhanced bondability so as to increase the strength of products that incorporate these fibers. The present invention seeks to fulfill these needs and offers further related advantages.
In one aspect, the present invention provides carboxylated cellulosic fibers. Fibrous sheets and absorbent products containing carboxylated cellulosic fibers are also disclosed. The fibrous sheets generally include carboxylated fibers, a cationic additive, and, optionally, other fibers.
In another aspect of the invention, a method for producing carboxylated cellulosic fibers is provided. The method produces carboxylated cellulosic fibers by applying a carboxylating agent to the fibers and then heating the treated fibers for a period of time under controlled temperature, time, pH, and catalyst concentration conditions to effect bond formation between the carboxylating agent and the fiber while minimizing crosslinking reactions. The carboxylating agent is any chemical compound having two carboxylic acid groups separated by either two or three atoms such that the compound can form a cyclic 5- or 6-membered anhydride. Suitable carboxylating agents include succinic acid and succinic acid derivatives, phthalic acid, trimellitic acid, maleic acid, and itaconic acid and their derivatives. Bond formation between the carboxylating agent and the fiber is preferably the formation of a single ester bond between the carboxylating agent and the fiber and not the formation of extensive fiber crosslinks.